1. Field of the Invention
This invention relates to catalyst compositions, process wherein these compositions are used for the preparation of liquid hydrocarbons from synthesis gas, and process for the preparation of said catalysts. in particular, it relates to catalysts, and process wherein C.sub.10 + distillate fuels, and other valuable products, are prepared by reaction of carbon monoxide and hydrogen over cobalt catalysts wherein the metal is dispersed as a thin film on the outside surface of a particulate titania carrier or support.
2. The Prior Art
Particulate catalysts, as is well known, are normally formed by dispersing catalytically active metals, or the compounds thereof upon carriers, or supports. Generally, in making catalysts the objective is to disperse the catalytically active material as uniformly as possible throughout a particulate porous support, this providing a uniformity of catalytically active sites from the center of a particle outwardly.
Catalysts have also been formed by dispersing catalytically active materials upon dense support particles; particles impervious to penetration by the catalytically active materials. Ceramic or metal cores have been selected to provide better heat transfer characteristics, albeit generally the impervious dense cores of the catalyst particles overconcentrates the catalytically active sites within a reduced reactor space and lessens the effectiveness of the catalyst. Sometimes, even in forming catalysts from porous support particles greater amounts of the catalytic materials are concentrated near the surface of the particles simply because of the inherent difficulty of obtaining more uniform dispersions of the catalytic materials throughout the porous support particles. For example, a catalytic component may have such strong affinity for the support surface that it tends to attach to the most immediately accessible surface and cannot be easily displaced and transported to a more central location within the particle. Catalyst dispersion aids, or agents are for this reason often used to overcome this effect and obtain better and more uniform dispersion of the catalytically active material throughout the catalyst particles.
Fischer-Tropsch synthesis for the production of hydrocarbons from carbon monoxide and hydrogen is now well known, and described in the technical and patent literature. The earlier Fischer-Tropsch catalysts were constituted for the most part of non-noble metals dispersed throughout a porous inorganic oxide support. The Group VIII non-noble metals, iron, cobalt, and nickel have been widely used in Fischer-Tropsch reactions, and these metals have been promoted with various other metals, and supported in various ways on various substrates, principally alumina. Most commercial experience, however, has been based on cobalt and iron catalysts. The first commercial Fischer-Tropsch operation utilized a cobalt catalyst, though later more active iron catalysts were also commercialized. The cobalt and iron catalysts were formed by compositing the metal throughout an inorganic oxide support. An important advance in Fischer-Tropsch catalysts occurred with the use of nickel-thoria on kieselguhr in the early thirties. This catalyst was followed within a year by the corresponding cobalt catalyst, 100 Co:18 ThO.sub.2 :100 kiesel-guhr, parts by weight, and over the next few years by catalysts constituted of 100 Co:18 ThO.sub.2 :200 kieselguhr and 100 Co:5 ThO.sub.2 :8 MgO:200 kieselguhr, respectively. These early cobalt catalysts, however, are of generally low activity necessitating a multiple staged process, as well as low synthesis gas throughout. The iron catalysts, on the other hand, are not really suitable for synthesis gas conversion due to the high degree of water gas shift activity possessed by iron catalysts. Thus, more of the synthesis gas is converted to carbon dioxide in accordance with the equation: H.sub.2 +2CO.fwdarw.(CH.sub.2).sub.x +CO.sub.2 ; with too little of the synthesis gas being converted to hydrocarbons and water as in the more desirable reaction, represented by the equation: EQU 2H.sub.2 +.fwdarw.(CH.sub.2).sub.x +H.sub.2 O.
U.S. Pat. No. 4,542,122 by Payne et al, which issued Sept. 17, 1985, describes improved cobalt catalyst compositions useful for the preparation of liquid hydrocarbons from synthesis gas. These catalyst compositions are characterized, in particular, as cobalt-titania or thoria promoted cobalt-titania, wherein cobalt, or cobalt and thoria, is composited or dispersed upon titania, or titania-containing support, especially a high rutile content titania. U.S. Pat. No. 4,568,663 by Mauldin, which issued Feb. 4, 1986, also discloses cobalt-titania catalysts to which rhenium is added to improve catalyst activity, and regeneration stability. These catalysts have performed admirably well in conducting Fischer-Tropsch reactions, and in contrast to earlier cobalt catalysts provide high liquid hydrocarbon selectivities, with relatively low methane formation.
Recent European Publication No. 0 178 008 (based on Application No. 85201546.0 filed: 25.09.85) and European Publication No. 0 174 696 (based on Application No. 852011412.5, filed: 05.09.85), having priority dates 04.10.84 NL 8403021 and 13.09.84 NL 8402807, respectively, also disclose cobalt catalysts as well as a process for the preparation of such catalysts by immersion of a porous carrier once or repetitively within a solution containing a cobalt compound. The cobalt is dispersed over the porous carrier to satisfy the relation .SIGMA.V.sub.p /.SIGMA.V.sub.c &lt;0.85 and .SIGMA.V.sub.p /.SIGMA.V.sub.c &lt;0.55, respectively, where V.sub.c represents the total volume of the catalyst particles and V.sub.p the peel volumes present in the catalyst particles, the catalyst particles being regarded as constituted of a kernel surrounded by a peel. The kernel is further defined as one of such shape that at every point of the kernel perimeter the shortest distance (d) to the perimeter of the peel is the same, d being equal for all particles under consideration, and having been chosen such that the quantity of cobalt present in .SIGMA.V.sub.p is 90% of the quantity of cobalt present in .SIGMA.V.sub.c. These particular catalysts, it is disclosed, show higher C.sub.5 + selectivities than catalysts otherwise similar except that the cobalt component thereof is homogeneously distributed, or uniformly dispersed, throughout the carrier. Suitable porous carriers are disclosed as silica, alumina, or silica-alumina, and of these silica is preferred. Zirconium, titanium, chromium and ruthenium are disclosed as preferred of a broader group of promoters. Albeit these catalysts may provide better selectivities in synthesis gas conversion reactions vis-a-vis catalysts otherwise similar except the cobalt is uniformly dispersed throughout the carrier, like other cobalt catalysts disclosed in the prior art, the intrinsic activities of these catalysts are too low as a consequence of which higher temperatures are required to obtain a productivity which is desirable for commercial operations. Higher temperature operation however leads to a corresponding increase in the methane selectivity and a decrease in the production of the more valuable liquid hydrocarbons.
Productivity, which is defined as the standard volumes of carbon monoxide converted/volume catalyst/hour, is, of course, the life blood of a commercial operation. High productivities are essential in achieving commercially viable operations. However, it is also essential that high productivity be achieved without high methane formation, for methane production results in lower production of liquid hydrocarbons. Accordingly, an important and necessary objective in the production and development of catalysts is to produce catalysts which are capable of high productivity, combined with low methane selectivity.
Despite improvements, there nonetheless remains a need for catalysts capable of increased productivity, without increased methane selectivity. There is, in particular, a need to provide further improved catalysts, and process for the use of these catalysts in synthesis gas conversion reactions, to provide further increased liquid hydrocarbon selectivity, especially C.sub.10 + liquid hydrocarbon selectivity, with further reduced methane formation.
3. Objects
It is, accordingly, the primary objective of this invention to fill this and other needs.
It is, in particular, an object of this invention to provide further improved, novel supported cobalt catalyst compositions, and process utilizing such compositions for the conversion of synthesis gas at high productivity, and low methane selectivity, to high quality distillate fuels characterized generally as C.sub.10 + linear paraffins and olefins.
A further and more particular object is to provide novel, supported cobalt catalyst compositions, both promoted and unpromoted which approach, or meet the activity, selectivity and productivity of powdered catalysts but yet are of a size acceptable for commercial synthesis gas conversion operation.
A further object is to provide a process utilizing such catalyst compositions for the production from synthesis gas to C.sub.10 + linear paraffins and olefins, at high productivity with decreased methane selectivity.
Yet another object is to provide a process for the preparation of such catalysts.
4. The Invention
These objects and others are achieved in accordance with this invention embodying a supported particulate cobalt catalyst formed by dispersing the cobalt as a thin catalytically active film upon the surface of a particulate titania support or substantially titania containing support and preferably wherein the rutile:anatase ratio of the titania is at least about 3:2. This catalyst can be used to produce, by contact and reaction at reaction conditions with an admixture of carbon monoxide and hydrogen, a distillate fuel constituted principally of an admixture of linear paraffins and olefins, particularly a C.sub.10 + distillate, at high productivity, with low methane selectivity. This product can be further refined and upgraded to high quality fuels, and other products such as mogas, diesel fuel and jet fuel, especially premium middle distillate fuels of carbon numbers ranging from about C.sub.10 to about C.sub.20.
In accordance with this invention the catalytically active cobalt component is dispersed and supported upon the titania particles as a thin catalytically active surface layer, or film, ranging in average thickness from about 0.02 millimeters (mm) to about 0.20 mm, preferably from about 0.04 mm to about 0.20 mm, with the loading of the cobalt being sufficient to achieve the productivity required for viable commercial operations, e.g., a productivity in excess of about 150. The cobalt loading, expressed as the weight metallic cobalt per packed bulk volume of catalyst, that achieves this result is at least about 0.04 grams (g) per cubic centimeter (cc) and preferably at least about 0.05 g/cc. Higher levels of cobalt tend to increase the productivity further and an upper limit of cobalt loading is a function of cobalt cost, diminishing increases in productivity with increases in cobalt, and ease of depositing cobalt. A suitable range may be from about. 0.04 g/cc to about 0.15 g/cc, preferably about 0.05 g/cc to about 0.15 g/cc more preferably about 0.05 g/cc to about 0.09 g/cc. The feature of a high cobalt metal loading in a thin catalytically active layer located at the surface of the particles is essential in optimizing the activity, selectivity and productivity of the catalyst in producing liquid hydrocarbons from synthesis gas, while minimizing methane formation.
Metals such as rhenium, zirconium, hafnium, cerium, thorium and uranium, or the compounds thereof, can be added to cobalt to increase the activity and regenerability of the catalyst. Thus, the thin catalytically active layers, or films, formed on the surface of the titania or substantially titania-containing support particles, can include in addition to a catalytically active amount of cobalt, any one or more of rhenium, zirconium, hafnium, cerium, uranium, and thorium, or admixtures of these with other metals or compounds thereof. Preferred thin catalytically active layers, or films, supported on the titania or titania-containing support, thus include cobalt-rhenium, cobalt-zirconium, cobalt-hafnium, cobalt-cerium, cobalt-uranium, and cobalt-thorium, with or without the additional presence of other metals or compounds thereof.
A particularly preferred catalyst is one wherein cobalt, or cobalt and a promoter, is dispersed as a thin catalytically active film upon a carrier or support that is titania, TiO.sub.2, or a material that is substantially titania, and preferably at least 50% titania, and still more preferably at least 80% titania, in which the titania has a rutile:anatase weight ratio of at least about 3:2, as determined by ASTM D 3720-78: Standard Test Method for Ratio of Anatase to Rutile In Titanium Dioxide Pigments By Use of X-ray Diffraction. Generally, the catalyst is one wherein the titania has a rutile:anatase ratio ranging at least about 3:2 to about 100:1, or greater, and more preferably from about 4:1 to about 100:1, or greater. Where any one of rhenium, zirconium, hafnium, cerium, thorium, or uranium metals, respectively, is added to the cobalt as a promoter to form the thin catalytically active film, the metal is added to the cobalt in concentration sufficient to provide a weight ratio of cobalt:metal promoter ranging from about 30:1 to about 2:1, preferably from about 20:1 to about 5:1. Rhenium and hafnium are the preferred promoter metals, rhenium being more effective in promoting improved activity maintenance on an absolute basis, with hafnium being more effective on a cost-effectiveness basis. These catalyst compositions, it has been found, produce at high productivity, with low methane selectivity, a product which is predominately C.sub.10 + linear paraffins and olefins, with very little oxygenates. These catalysts also provide high activity, high selectivity and high activity maintenance in the conversion of carbon monoxide and hydrogen to distillate fuels.
The cobalt catalysts of this invention, as contrasted with (i) cobalt catalysts, the cobalt portion of which is uniformly distributed throughout the support particles or (ii) cobalt catalysts having a relatively thick surface layer of cobalt on the support particles, have proven especially useful for the preparation of liquid hydrocarbons from synthesis gas at high productivities, with low methane formation. In contrast with the catalysts of this invention, the prior art catalysts are found to have lower activity, and especially poorer selectivity due to a severe diffusion limitation. These catalysts (i) and (ii), supra, at high productivities, produce altogether too much methane. As productivity is increased to produce greater conversion of the carbon monoxide to hydrocarbons, increased amounts of methane are concurrently produced. Thus, increased productivity with these catalysts could only be obtained at the cost of increased methane formation. This result occurs, it is believed, because the carbon monoxide and hydrogen reactants all too slowly diffuse through the pores of the particulate catalyst which becomes filled with a liquid product, thus resulting in underutilization of the catalytically active sites located within the interior of the particles. Both hydrogen and carbon monoxide must diffuse through the product-liquid filled pores, but hydrogen diffuses through the pores at a greater rate of speed than the carbon monoxide. Since both the hydrogen and the carbon monoxide are reacting at the catalytic sites at an equivalent rate, a high H.sub.2 /CO ratio is created in the interior of the particle which leads to high methane formation. As the rate of reaction is increased, e.g., by incorporating higher intrinsic activity or by operating at higher temperature, the catalyst becomes more limited by the rate of diffusion of the reactants through the pores. Selectivities are especially poor under the conditions of high productivity. Thus, the catalyst used during a Fischer-Tropsch hydrocarbon synthesis reaction is one the pores of which become filled with the product liquid. When the CO and H.sub.2 are passed over the bed of catalyst and consumed at a rate which is faster then the rate of diffusion, H.sub.2 progresses to the interior of the particle to a much greater extent than the CO, leaving the interior of the particles rich in H.sub.2, and deficient in CO. The formation of methane within the particle interior is thus favored due to the abnormally high H.sub. 2 /CO ratio; an unfavorable result since CH.sub.4 is not a desirable product. The extent to which selectivity is debited depends on the magnitude of the difference between the rate of diffusion and the rate of reaction, i.e., the productivity.
The catalyst of this invention is thus one wherein essentially all of the active cobalt is deposited on the surface of the titania or titania-containing support particles. The surface film of cobalt must be very thin and contain an adequate loading of cobalt to maximize reaction of the hydrogen and carbon monoxide at the surface of the catalytic particle. The surface film of cobalt as stated thus ranges generally from about 0.02 mm to about 0.20 mm, preferably from about 0.04 mm to about 0.20 mm, with cobalt loadings at least about 0.04 g/cc, preferably at least about 0.05 g/cc, more preferably ranging from about 0.04 g/cc to about 0.15 g/cc, still more preferably from about 0.05 g/cc to about 0.15 g/cc,and even more preferably ranging from about 0.05 g/cc to about 0.09 g/cc, calculated as metallic cobalt per packed bulk volume of catalyst. The promoter metal to be effective must also be contained within the surface film of cobalt. If extended into the interior of the particle outside the cobalt film the promoter metal will have little promotional effect, if any. The metal promoter should thus also be concentrated within the cobalt film at the surface of the catalyst, with the weight ratio of cobalt:metal promoter, as suggested, ranging from about 30:1 to about 2:1, preferably from about 20:1 to about 5:1. The thickness of the surface metal film can be conveniently measured by an Electron Probe Analyzer, e.g., one such as produced by the JEOL Company, Model No. JXA-50A. Cross-sections of the catalyst particles of this invention measured via use of this instrument show very high peaks, or shoulders, at the edges of the particle across the line of sweep representative of cobalt concentration, with little or no cobalt showing within the particle interior. The edge, or "rim" of the "radially impregnated catalyst" will thus contain essentially all of the cobalt added to the catalyst. The thickness of the film, or rim, is unrelated to the absolute size, or shape of the support particles. Virtually any size particle can be employed as is normally employed to effect catalyst reactions of this type, the diameter of the particle ranging generally from about 0.5 mm to about 2 mm. The particles can be of virtually any shape, e.g., as is normally employed to effect reactions of this type, viz., as beads or spheres, extrudates, saddles or the like. By concentrating the catalytic metal, or metals, on the extreme outer surface of the particles, the normal diffusion limitation of the catalyst can be minimized to the extent that diffusion limitation is no longer a deleterious problem. This new catalyst is more active in its function of bringing about a reaction between the CO and H.sub.2. The catalyst because of its having the thin layer of catalytically active metal on its surface is in effect found to behave more ideally, approaching, in fact, the behavior of a powdered catalyst which does not exhibit diffusion limitations. However, unlike the use of powdered catalysts, the flow of the reactants through the catalyst bed is virtually unimpeded. Higher productivity, with lower methane selectivity, is the result; a result of considerable commercial consequence. At productivities (at 200.C) greater than 150 hour.sup.-1 (standard volumes of carbon monoxide converted per volume of catalyst per hour), notably from about 150 hour.sup.-1 to about 200 hour.sup.-1, less than 10 mole percent of the carbon monoxide converted is converted to methane.
In conducting synthesis gas reactions the total pressure upon the CO and H.sub.2 reaction mixture is generally maintained above about 80 psig, and preferably above about 140 psig. It is generally desirable to employ carbon monoxide, and hydrogen, in molar ratio of H.sub.2 :CO above about 0.5:1 and preferably equal to or above about 1.7:1 to increase the concentration of C.sub.10 + hydrocarbons in the product. Suitably, the H.sub.2 :CO molar ratio ranges from about 0.5:1 to about 4:1, and preferably the carbon monoxide and hydrogen are employed in molar ratio H.sub.2 :CO ranging from about 1.7:1 to about 2.5:1. In general, the reaction is carried out at gas hourly space velocities ranging from about 100 V/Hr/V to about 5000 V/Hr/V, preferably from about 300 V/Hr/V to about 1500 V/Hr/V, measured as standard volumes of the gaseous mixture of carbon monoxide and hydrogen (0.degree. C., 1 Atm.) per hour per volume of catalyst. The reaction is conducted at temperatures ranging from about 160.degree. C. to about 290.degree. C., preferably from about 190.degree. C. to about 260.degree. C. Pressures preferably range 80 psig to about 600 psig, more preferably from about 140 psig to about 400 psig. The product generally and preferably contains 60 percent, or greater, and more preferably 75 percent, or greater, C.sub.10 + liquid hydrocarbons which boil above 160.degree. C. (320.degree. F.).
The catalysts employed in the practice of this invention can be prepared by spray techniques where a solution of a cobalt compound, alone or in admixture with a promoter metal compound, or compounds as a spray is repetitively contacted with hot titania, or titania-containing support particles. The particulate titania or titania-containing support particles are preheated to temperatures equal to or above about 140.degree. C. and then contacted with the spray. Suitably the temperature of the titania, or titania-containing support, ranges from about 140.degree. C. up to the decomposition temperature of the cobalt compound, or compounds in admixture therewith; preferably from about 140.degree. C. to about 190.degree. C. The cobalt compound employed in the solution can be any organometallic or inorganic compound which decomposes to give cobalt oxide upon initial contact or upon calcination, such as cobalt nitrate, cobalt acetate, cobalt acetylacetonate, cobalt naphthenate, cobalt carbonyl, or the like. Cobalt nitrate is especially preferred while cobalt halide and sulfate salts should generally be avoided. The cobalt salts may be dissolved in a suitable solvent, e.g., water, organic or hydrocarbon solvent such as acetone, methanol, pentane or the like. The total amount of impregnation solution used should be sufficient to supply the proper catalyst loading, with the film being built up by repetitive contacts between the support and the solution. The preferred catalyst is one which consists essentially of cobalt, or cobalt and promoter, dispersed upon the titania, or titania-containing support, especially a rutile support. Suitably, the hot titania support is contacted with a spray which contains from about 0.05 g/ml to about 0.25 g/ml, preferably from about 0.10 g/ml to about 0.20 g/ml, of the cobalt compound or cobalt compound plus the compound containing the promoter metal, generally from at least about 3 to about 12 contacts, preferably from about 5 to about 8 contacts, with intervening drying and calcination steps being required to form surface films of the required thicknesses. The hot titania, or titania-containing support, in other words, is spray-contacted in a first cycle which includes the spray contact per se with subsequent drying and calcination, a second cycle which includes the spray contact per se with subsequent drying and calcination, a third spray contact which includes the spray contact per se with subsequent drying and calcination, etc. to form a film of the required thickness and composition. The drying steps are generally conducted at temperatures ranging above about 20.degree. C., preferably from about 20.degree. C. to about 125.degree. C., and the calcination steps at temperatures ranging above about 150.degree. C., preferably from about 150.degree. C. to about 300.degree. C.
Titania is used as a support, either alone or in combination with other materials for forming a support, but the titania preferably makes up at least about 50% of the support. The titania used for the support is preferably one which contains a rutile:anatase ratio of at least about 3:2, as determined by x-ray diffraction (ASTM D 3720-78). The titania preferably has a rutile:anatase ratio of from about 3:2 to about 100:1, or greater, more preferably from about 4:1 to about 100:1, or greater. The surface area of such forms of titania are less than about 50 m.sup.2 /g. These weight concentrations of rutile provide generally optimum activity, and C.sub.10 + hydrocarbon selectivity without significant gas and CO.sub.2 make. The prepared catalyst as a final step is dried by heating at a temperature above about 20.degree. C., preferably between 20.degree. C. and 125.degree. C., in the presence of nitrogen or oxygen, or both, in an air stream or under vacuum. It is necessary to activate the catalyst prior to use. Preferably, the catalyst is contacted with oxygen, air, or other oxygen-containing gas at temperature sufficient to oxidize the cobalt and convert the cobalt to Co.sub.3 O.sub.4. Temperatures ranging about about 150.degree. C., and preferably above about 200.degree. C. are satisfactory to convert the cobalt to the oxide, but temperatures above about 500.degree. C. are to be avoided unless necessary for regeneration of a severely deactivated catalyst. Suitably, the oxidation of the cobalt is achieved at temperatures ranging from about 150.degree. C. to about 300.degree. C. The metal, or metals, contained on the catalyst are then reduced. Reduction is performed by contact of the catalyst, whether or not previously oxidized, with a reducing gas, suitably with hydrogen or a hydrogen-containing gas stream at temperatures above about 200.degree. C.; preferably above about 250.degree. C. Suitably, the catalyst is reduced at temperatures ranging from about 200.degree. C. to about 500.degree. C. for periods ranging from about 0.5 to about 24 hours at pressures ranging from ambient to about 40 atmospheres. A gas containing hydrogen and inert compounds in admixture is satisfactory for use in carrying out the reduction.
The catalysts of this invention can be regenerated, and reactivated to restore their initial activity and selectivity after use by washing the catalyst with a hydrogen solvent, or by stripping with a gas. Preferably the catalyst is stripped with a gas, most preferably with hydrogen, or a gas which is inert or non-reactive at stripping conditions such as nitrogen, carbon dioxide, or methane. The stripping removes the hydrocarbons which are liquid at reaction conditions. Gas stripping can be performed at substantially the same temperatures and pressures at which the reaction is carried out. Pressures can be lower however, as low as atmospheric or even a vacuum. Temperatures can thus range from about 160.degree. C. to about 290.degree. C., preferably from about 190.degree. C. to about 260.degree. C., and pressures from below atmospheric to about 600 psig, preferably from about 140 psig to about 400 psig. If it is necessary to remove coke from the catalyst, the catalyst can be contacted with a dilute oxygen-containing gas and the coke burned from the catalyst at controlled temperature below the sintering temperature of the catalyst. Most of the coke can be readily removed in this way. The catalyst is then reactivated, reduced, and made ready for use by treatment with hydrogen or hydrogen-containing gas as with a fresh catalyst.
The invention will be more fully understood by reference to the following examples and demonstrations which present comparative data illustrating its more salient features.
The catalysts of this invention are disclosed in the following examples and demonstrations as Catalysts Nos. 14-21. These are catalysts which have surface films falling within the required range of thicknesses, and the surface film contains the required cobalt metal loadings. It will be observed that all of Catalysts Nos. 14-21 were formed by a process wherein a heated particulate TiO.sub.2 substrate was repetitively contacted with a dilute spray solution containing both the cobalt and rhenium which was deposited as a thin surface layer, or film, upon the particles. Catalysts Nos. 14-21 are contrasted in a series of synthesis gas conversion runs with Catalysts Nos. 1-8, catalysts wherein the metals are uniformly dispersed throughout the TiO.sub.2 support particles. They are also contrasted in runs made with Catalysts Nos. 9-13, also "rim" catalysts but catalysts wherein the surface films, or rims, are either too thick (Catalysts Nos. 11-13), however prepared, or do not contain an adequate cobalt metal loading within the surface film, or rim (Catalyst Nos. 9 and 10). From the data presented, at high productivities the catalysts formed from the uniformly impregnated TiO.sub.2 spheres produce high methane. Moreover, even wherein a film of the catalytic metal is formed on the surface of the particles, it is essential that the surface film, or rim of cobalt be very thin and also contain an adequate loading of cobalt in the film. This is necessary to maximize reaction of the H.sub.2 and CO at the surface of the particle wherein the cobalt metal reaction sites are located, while simultaneously reactions within the catalyst but outside the metal film or rim are suppressed to maximize productivity, and lower methane selectivity. The following data thus show that the catalysts of this invention, i.e., Catalysts Nos. 14-21, can be employed at productivities above 150 hour.sup.-1 to 200 hour.sup.-1, and greater, to produce no more and even less methane than is produced by (i) catalysts otherwise similar except that the catalysts contain a thicker surface film, i.e., Catalysts Nos. 11-13, or (ii) catalysts which contain an insufficient cobalt metal loading within a surface film of otherwise acceptable thinness, i.e., Catalyst Nos. 9 and 10. The data show that the catalysts of this invention at productivities ranging above about 150 hour.sup.-1 to about 200 hour.sup.-1, and greater, can be employed to produce liquid hydrocarbons at methane levels well below 10 mole percent.